Searching for Regional Crustal Velocity-Density Relations with the Use of 2-D Gravity Modelling – Central Europe Case
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In this paper we search for a reference relation between seismic P-wave velocity V and density ρ ref for the continental crust. Based on the results of modern seismic experiments, we compiled 2-D seismic models into a network of four, each about 1100–1400 km long, continental-scale seismic transects cutting all main tectonic units in Central Europe. The Moho depth (about 52 km beneath the TESZ in SE Poland, to about 25 km beneath the Pannonian Basin) and the crustal structure of this area are characterised by a large variation. This structural variation provides an interesting basis for gravity studies and especially for analysing the difference of the density structure between two major tectonic provinces of distinctive age difference: Precambrian and Phanerozoic. The 2-D gravity modelling applied for crustal cross-sections representing the regional structure, based on a unified gravity anomaly map of the area, allows for a stable determination of some general features of the regional reference velocity-density relation for the continental crust. In general three major seismo-petrological types of rocks can be distinguished: sediments, crystalline crust and mantle. In compacted sediments the reference velocity-density relation is well described by the Gardner or Nafe-Drake model. Calculated gravity anomalies, using unified velocity-density relation for the whole crystalline crust, well describe observed anomalies, with an average difference of 14 mGal. However, calculated gravity anomalies, using separated velocity-density relations for the crystalline crust of Precambrian and Phanerozoic Europe, describe observed anomalies better than for the entire crust, with an average difference 12 mGal. The most important feature of these relations is the large differentiation of the derivative dρ ref /dV in the crystalline crust, being about 0.3 g s/m4 for Precambrian, and about 0.1 g s/m4 for the Phanerozoic crystalline crust. The modelling suggests a very small density value in the uppermost mantle ρ = 3.11 g/cm3 below the younger area, while for the older area it is ρ = 3.3 g/cm3.